1. Field of the Invention
The present invention relates to diode power sensors. More particularly, the present invention relates to diode power sensors designed to measure power over a wide dynamic range.
2. Description of the Related Art
Diode power sensors take advantage of a square law operating region of a diode to measure power. The current (I) vs. voltage (V) equation for a diode is typically expressed in an exponentional form according to the equation EQU I=Io(e.sup.(.eta.V/kT) -1) (1)
where Io and .eta. are constants whose values depend on the details of the diode, T is the diode's temperature in Kelvins and k is Boltsmann's constant. FIG. 1 shows an ideal I-V curve for a Schottky or PN-Junction type diode following the parameters of equation (1).
A diode's I-V behavior can also be represented in terms of a polynomial series, ##EQU1##
where the a.sub.n values are chosen to suit a particular diode being considered. For a diode receiving a sinusoidal voltage, average current can be calculated using the second order term of equation (2), while other terms of equation (2) can be neglected over a portion of the diode I-V curve. Equation (2) can, thus, be simplified for average sinusoidal current as: EQU I.sub.avg =a(V.sup.2).sub.avg (3)
The portion of the I-V curve where a diode operates according to equation (3) is referred to as the square law region.
A power sensor which measures RMS power can be constructed using a Schottky or PN-Junction diode to take advantage of the I-V square law relation of the diode. RMS power can then be determined by measuring average diode current. Power is determined according to the equation EQU P=(V.sup.2).sub.avg /2R (4)
where P is average power, and R is the load resistance of typically 50 .OMEGA.. Measured average current I.sub.avg from a diode is related to the average of the square of the diode voltage V according to equation (3), and the average of the square of V is related to average power using equation (4). Therefore average power P can be determined from average diode current I.sub.avg according to the equation EQU P=I.sub.avg /2aR (5)
The power range over which a real diode operates according to the square law is limited. The typical square law operating range for a real diode is approximately -70 dBm to -20 dBm. An alternative method for measuring true RMS power is to use more expensive peak sensor/meter systems. The signal channel for these peak sensor/meter systems faithfully follows the voltage envelope for a modulated signal. Measured voltage values along the envelope are averaged to get true RMS average power. The sensor/meter systems are limited because measurements from a signal that is modulated at a higher frequency than a sensor/meter system bandwidth will produce an inaccurate measurement.